Smart antennas are antenna arrays installed at base station (BTS) sites and are one of the key features of TD-SCDMA (Time Division-Synchronous Code Division Multiple Access), which was developed in China. TD-SCDMA, as one of the three 3rd generation (3G) wireless communication system standards, uses TDMA together with CDMA as multiple access method, especially it adopts TDD (Time Division Duplex) as its duplex mode, which provides rather convenience for application of smart antennas. In a typical TD-SCDMA configuration, the antenna array of the smart antenna is a circular array composed of eight antenna elements. The TD-SCDMA smart antenna system combines the multiple antenna elements from the antenna array using beam forming concepts and other sophisticated signal processing algorithms to transmit and receive adaptively.
In general, a smart antenna system is composed of N antenna elements with N related feed cables and N coherent RF (Radio Frequency) transceivers in the RF part. It serves to provide beam forming which points to a particular terminal device (or user equipment (UE) in 3G terminology). Thus, instead of “illuminating” the entire cell with radio power, the base station sends power only to the terminals that are active in the cell. This illumination has the immediate benefit of increasing the power received by the terminals in the cell' while reducing mobile-to-mobile interference in the cell and interference in nearby cells.
Smart antenna systems in TD-SCDMA and TDD systems can be very effective because these systems use the same frequency for both the uplink and the downlink and therefore can assume a nearly identical channel in both directions. Smart antennas are therefore widely recognized as a promising technology to address the demand of wireless communication systems capacity and coverage when employed in place of traditional antenna to reduce non-desired user interference from space domain.
Smart antennas are usually categorized as either a switched-beam or adaptive array antennas. Compared with adaptive array antennas, switched-beam antennas are less complex and easier to implement, which is attractive to either manufacturers or operators. Furthermore, switched-beam smart antenna systems offer a robust implementation against multi-path propagation effects and reduced complexity that is inherent with fully adaptive smart antenna implementations.
However, one of the main limitations of switched-beam smart antennas is the phenomenon of “scalloping”, which means that the power received from one UE by one BTS, which employs switched beam smart antennas, is fluctuant when the UE moves around the BTS. When the DOA (Direction Of Arrival) is diverging from the axis of the selected beam, the available antenna gain decreases.
FIG. 2 shows an example of a fixed antenna pattern of a conventional smart antenna with a plurality of fixed beams. In FIG. 2, UE1 is located at the intersection area of beam A and beam B, while UE2 and UE3 are located at the maximum gain direction of beam A and beam B. No matter whether UE1 selects beam A or beam B, its signal would be discriminated against UE2 or UE3 because the antenna gain for UE1 at the intersection area of two beams is much less than the maximum antenna gain of beam A or beam B. Hence, UE 1 cannot make use of the maximum antenna gain from either UE2 or UE3's beam. Then, if considering UE1, the interference of UE2 or UE3 might have higher antenna gain than the signal received from UE1, which is not what should be expected from the effect of the switched-beam smart antennas.
For switched-beam smart antennas, the beam pattern and the beam width are typically fixed and cannot be unlimited narrow because of the limitations on implementation cost and hardware feasibility. Thus, when a user happens to be not located around a beam peak area, this “scalloping” problem will be unavoidable.
Document US20020068590A1 discloses a wireless communication method and system using an antenna array with variable beam direction. In particular, the beam direction of a particular beam is controlled to vary in different time slots. In order to prevent radio waves from respective base stations from interfering with each other, they are controlled to be radiated at different times, thereby avoiding interference. To achieve this, the operations of the different base stations are managed to be synchronized with each other, so that the directions in which the base stations radiate radio waves can be timely switched to avoid interference. However, this prior art does serves to prevent beam interference but not the above scalloping problem.